1. Field of the Invention
The present invention relates to a process for preparing rhombohedral system boron nitride (hereinafter referred to simply as "rBN"). More particularly, it relates to a process for producing a boron nitride powder containing rBN at a high concentration.
2. Description of the Prior Art
Rhombohedral system boron nitride (rBN) is important as a starting material for the synthesis of cubic system boron nitride (hereinafter referred to simply as "cBN") by a shock compression method. The presence of hexagonal system boron nitride (hereinafter referred to simply as "hBN") in the starting material is likely to lead an inclusion of wurtzite structure boron nitride (hereinafter referred to simply as "wBN") in the compression product, and the presence of irregularly layer-structured boron nitride tends to decrease the yield of cBN. Accordingly, it is desired to decrease the content of hBN in the starting material.
Heretofore, the preparation of boron nitride has been conducted by (1) a method wherein a mixture of boric acid or borax with potassium cyanide is heated, or (2) a method wherein boron oxide is vaporized, and then reduced and nitrided by e.g. hydrogen cyanide. In the method (1), rBN is obtainable as a mixture with hBN. In the method (2), highly pure rBN is obtainable. However, this method requires expensive installations for the vaporization of boron oxide, and consequently the production cost will be high. Furthermore, these conventional methods require the use of a highly toxic reagent such as potassium cyanide or hydrogen cyanide.